Crystalline thermoplastic polyester resin, for example, polyalkylene terephthalate resins including polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), are typically employed as engineering resins in numerous end-use applications due to their desirable physical and chemical properties (e.g., mechanical strengths, electrical properties, processability, etc.). Thus, crystalline thermoplastic polyester resins have been employed in the past to form molded components and parts in the automotive and electronics industries.
Polyalkylene terephthalate resins as a class generally exhibit relatively high rigidity, and are sometimes modified in this respect by melt-blending relatively more "flexible" resins. Of course, when resins are melt-blended, the properties of the blend represents a merger of the resin components forming the blend. Thus, in those end-use applications which require the resulting article to exhibit discrete portions or regions of rigidity and flexibility, modified melt-blends of polyalkylene terephthalate resins cannot be employed.
Molding techniques are known, however, that allow two (or more) discrete resins to be molded so that the resulting article will be a composite of all resins that were employed in the molding operation--i.e., the composite article will have discrete portions or regions corresponding respectively to the resins that are molded. These techniques are typically known as "double" or "two-color" molding methods and generally contemplate molding a preform of a desired article from a primary resin, and then molding a secondary resin onto the primary resin preform such that the primary and secondary resins contact one another along an interfacial boundary therebetween.
The interfacial molding techniques described above, however, sometimes do not produce composite articles of satisfactory quality and/or physical characteristics since it is difficult to fuse the primary and secondary resins one to another along their boundary interface. A composite article in which the primary and secondary resins are insufficiently fused or bonded is problematic since the respective portions of the article formed of the primary and secondary resins may separate or "peel" from one another along the interface boundary when subjected to external forces (e.g., loads during operation). As a result, such an article may warp to such an extent that it becomes unusable in its intended application.
In order to improve the interfacial bonding between dissimilar resins (e.g., resins having relatively high rigidity and flexibility), it has been proposed to include mechanical anchoring structures (e.g. an undercut or aperture region) along the interface boundary between the primary and secondary resins. In addition, adhesives have been applied along the interface boundary between two dissimilar resins in an attempt to provide enhanced bonding. As can be appreciated, however, the use of mechanical anchoring structures and/or adhesives in an effort to enhance the bonding efficacy between two dissimilar resins increases the complexity (and hence costs) of the article and/or the process for molding the same. As a result, composite articles formed by interfacial molding techniques are not typically capable of being produced economically and/or efficiently.
Specifically, interfacial molding techniques usually cannot be employed with highly crystalline polyalkylene terephthalate resin having a relatively high rigidity and a relatively more flexible resin to produce efficiently and economically a composite molded article having discrete regions of rigidity and flexibility attributable to the respective resins. Moreover, it is difficult to form a composite molded article of relatively highly rigid and flexible resins whereby the resins are sufficiently bonded one to another along an interfacial boundary. Thus, what has been needed are techniques to allow relatively rigid and flexible resins to be molded so as to form composite articles having desirably high bonding strength therebetween. It is towards achieving such needed molding techniques that the present invention is directed.
According to the present invention, molding techniques are provided which allow composite articles having discrete regions of relatively high rigidity and flexibility are provided. More particularly, composite molded articles having enhanced fusion strength at the interfacial boundary between one region formed of a relatively highly rigid resin and one region formed of a relatively flexible resin are provided according to this invention using a simple interfacial molding technique. Moreover, these composite articles can be formed without resorting to use of mechanical anchoring structures and/or all adhesive along the boundary interface. That is, the enhanced fusion between the relatively rigid and flexible regions of the composite article is achieved solely by means of the fusion between the relatively rigid and flexible resins along the interfacial boundary.
More specifically, according to the present invention, composite molded articles having relatively rigid and flexible resin portions are bonded to one another along an interfacial fusion zone. A normally solid thermoplastic polyalkylene terephthalate resin preform (which will constitute the relatively rigid position of the composite article) is first placed in a mold cavity. The flexible portion of the composite article is then formed by introducing a molten thermoplastic polyester elastomer resin into the mold cavity at a temperature which is greater than the melt temperature of the solid preform. The introduced thermoplastic polyester elastomer resin contacts and at least partially plasticizes a surface region of the solid preform along the interfacial bonding zone within the mold cavity to thereby cause the polyester elastomer resin to coalesce with the at least partially plasticized surface region of the polyester resin. Upon cooling and solidification, the solid preform and the polyester elastomer are thereby fused to one another along the interfacial bonding zone.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follow.